Aqueous acrylic resin electrodeposition coating composition containing a mixture of organic amines

ABSTRACT

COMPOSITION CONTAINING AN ACRYLIC RESIN HAVING CARBOXYLIC GROUPS, AT LEAST ONE ORGANIC AMINE HAVING A &#34;LOWER&#34; DISSOCIATION DEGREE OF ABOVE 6 PKB, AND AT LEAST ONE ORGANIC AMINE HAVING A &#34;HIGHER&#34; DISSOCATION DEGREE, I.E. ITS PKD IS LESS THAN THAT OF THE ABOVE-DESCRIBED ORGANIC AMINE BY TWO OR MORE, IS SUITABLE FOR ELECTRODEPOSITION COATING TO YIELD AN EXCELLENT COATING FILM.

United States Patent Ofice 3,794,607 Patented Feb. 26, 1974 Int. or. cost 15/40, 45/60 US. Cl. 26029.6 TA 4 Claims ABSTRACT OF THE DISCLOSURE Composition containing an acrylic resin having carboxylic groups, at least one organic amine having a lower dissociation degree of above 6 pKb, and at least one organic amine having a higher dissociation degree, i.e. its pKb is less than that of the above-described organic amine by two or more, is suitable for electrodeposition coating to yield an excellent coating film.

This is a continuation of Ser. No. 812,437, filed Apr. 1, 1969, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to an acrylic coating composition for electrodeposition. Many compositions for electrodeposition are well known. Resins with a carboxyl group and which are aqueous-solubilized with various amines, ammonium materials, and other bases and which are often mixed with other resins, pigments, and other additives, are commonly used.

in general, basic resins of an acrylic composition used for electrodeposition employs a resin as the base which results from the copolymerization of a copolymerizable acid, such as acrylic acid, methacrylic acid or itaconic acid, with acrylate esters, methacrylate esters, styrene, or vinyl toluene, etc. by radical polymerization. If necessary, though the resin may occasionally be co-condensated or reacted with various resins or materials, and this is the situation in most cases, the resin can be water solubilized by neutralizing a part or all of the carboxylic acid therein with various amines, ammonia, or other bases. However, in practical usage, the thermosetting acrylic resin composition used for electrodeposition which employs these acrylic resins as the base has the defects common to the acrylic compositions, i.e., poor flowability at the curing temperature with the resulting poor surface characteristics such as orange peel. (At least in comparison with the use of thermosetting alkyd or oily compositions.)

One conventional method for improving the surface appearance of such a coating is to add a high boiling point solvent, plasticizer, etc. thereto. However, the addition of such materials is not preferred, since they soften the coating film.

An object of the present invention is to improve the surface appearance acrylic coating composition without damaging the characteristics of coating, that is, to provide an acrylic coating composition for use in an electrodeposition coating process which is capable of providing a coating film having an excellent surface appearance. This is basically accomplished by improving a flowability of coating at the high temperature.

SUMMARY OF THE INVENTION The. composition of the present invention is an acrylic coating composition useful for electrodeposition which contains an acrylic resin having a carboxylic group, at least one organic amine compound having a lower dissociation constant of above 6 pKb, and at least one organic amine compound having a higher dissociation constant, i.e., less than that of the above-described organic amine compound in its pKb by more than two (as a water solubilizing agent for the above-described resin pKb=log Kb; Kb: dissociation constant of amine).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Any method may be selected for the addition of the organic amine compound. That is, the basic resin of the composition is prepared by polymerization and condensation, and, subsequently, upon water-solubilizing the resin, the above-described two amines may be added simultaneously. Other amines may be added before or after or during addition of the water-solubilizing agent. Also, when employing the composition in electrodeposition coating, the composition can be prepared in the vessel used for coating by adding the above-described two amines thereto in any order.

The organic amine compounds which can be employed in the present invention are exemplified as follows (remembering that pKb=log Kb, with Kb being the dissociation constant of the amine).

Organic amine compound of higher dissociation constant: pKb Triethylarnine 3.4 Trimethylamine 4.2 Tri-n-propylamine 3.4 Tributylamine n 3.1, iso 3.7 Diethylamine 4.0 Diethylaminoethanol 3.5-4.5 Triethanolamine 6.2 Methylamine 3.4 Ethylamine 3.4 n-Propylamine 3.4 Hexylamine 34.5 Morphorine 3-5 .0 Piperidine 34.5 Dimethylamine 3.1 Diethylamine 3.1 D-n-propylamine 3.0

Primary and secondary amines are not as preferable because of the possibility of their injuring the stability of the system.

Organic amine compound of lower dissociation constant: pKb a-Picoline 8.8 B-Picoline 8.0 'y-Picoline 8.3 2,6-1utidine 7.3 2,4-lutidine 7.2 3,5-lutidine 77.5 N-methylmorpholine 6.3 Pyridine 8.8 Quinoline Above 6.5

The pKb of the organic amine compound having higher dissociation constant is preferably 3.0 to 6.3, and the pKb of that having lower dissociation constant is preferably 6 to 9.

The amounts of organic amine compound to be added vary depending upon the kind of resin and the ratio of resin to composition.

The organic amine compound of lower dissociation degree scarcely affects the water-solubilization of the resin and the pH of the composition. These parameters are substantially determined by the organic amine compound of higher dissociation degree. Therefore, the amount of organic amine compound of higher dissociation degree used should be added so that water-solubilization is sufficient and the pH is in the range of 6 to 10 (considering the performance of the coating finally obtained) and the resin is not softened (in comparison with the case of from 0.3 to 1.2 equivalent amounts based on thecarusing a plasticizer).

boxylic acid contained in the acrylic resin are commonly The effect of the present invention is considered to be used. When below 0.3 equivalent are used, the pH of the attributable to the fact that the amine of lower dissociasystem becomes too high to be suitable for electrodep- 5 tion degree not only functions in a similar manner to a osition. The organic amine compound of low dissociasolvent, but also serves to appropriately slow down the tion degree, which scarcely affects the pH in practical acid catalyst reaction which is a curing reaction of such commercial usage is employed in an amount of from 0.1 resins. This thereby gives time enough for the composito 30%, preferabl 0.2 to 20%, by weight, of the solid tion to flow sufiiciently and to give a smooth coating surcontent of the acrylic composition. The effect of improv- 1O face during curing.

ing the smoothness of surface is not recognized in an The present invention will be illustrated with the folamount below 0.1% by weight, and also, with above 30% lowing examples in which part means part by weight. by weight, the electrodeposition characteristics will be EXAMPLE 1 changed, and the electrodeposited film is softened, that is, for example, in water-washing before baking a water 16 parts of styrene, 58 parts of butyl acrylate, 15 parts trace will remain to damage the surface of the coating of y y y methacl'ylate, 11 Parts of itacohic acid, film, 60 parts of isopropylalcohol (as a solvent) and 1.5 parts As the base of the composition, acrylic resins are prefof benzoylperoxide (as a polymerization initiator) were erably employed which are prepared by polymerizing 3 heated, under reflux, with stirring, and were polymerized. to 20 parts, by weight of a carboxylic acid having a co- 20 Two parts of pyridine were added thereto and triethylpolymerizable double bond, such as, acrylic acid, methamine was further added so that the pH of the aqueous acrylic acid, itaconic acid, etc., 4 to 97 parts, by weight, resin solution (10% by weight) was 8.5. Hexamethoxy of an ester thereof, and 0 to 40 parts, by weight, of a resin melamine [hexakis'(methoxymethyl)melamine], Cymaterial co-polymerizable therewith, for example, acrylmel 300 (trademark), was added thereto so that the solid amide, styrene, vinyl toluene, etc. content ratio of acrylic resin to melamine was 7 to 3.

An acrylic coating composition may be manufactured Water was then added to adjust the solid content to 10%, by water-solubilizing the above-described acrylic resin by weight. This liquid was maintained at 28 C. and was and adding various additives thereto, although the waterelectrodeposited on a zinc phosphate treated iron plate solubilization can be carried out after the addition of the under the conditions of 50 volts for one minute. After various additives. The additives used include various resins 30 washing with water, the coated member was cured at such as melamine resins, epoxidc resins, etc.; various pig- 150 C. for 20 minutes to obtain a coating material havments, various organic solvents, and plasticizers, etc. ing a smooth surface. The coating was H grade in pencil These are added, usually only if necessary. hardness and was excellent in alkali resistance, water re- The composition of the present invention can be emsistance and various other physical properties. ployed in any conventional method commonly well known The results of various experiments to determine the as coating methods utilizing the electrophoresis phenomeeffects of substituting other substances for pyridine and non, that is, the so-called electrodeposition coating methvarying the water-solubilizing agent are shown in Table 1. 0d. The resin of Example 1 was used.

TABLE 1 No.1 No.2 No.3 No.4 No.5

Water'solubilizing agent Triethylamine Trlethylamine T1iethy1amine Triethylamine- Triethylarnine. Additive- Pyridine None Di-octylphthalate. n-Butanol None. Amount of additive, part/100 parts of acryl 2 Surface smoothness Hardness (pencil) Alkali resistance, 1% NaOH for 1 day Impact strength, 500 g./cm. in.)

Coating voltage Excellent i 00! Fair Fair Poor. H H H.

1 The composition of the present invention.

2 Using only the water solubilizing agent as an additive.

3 Using a plasticizer as an additive.

4 Using a solvent as an additive.

5 Using two organic amines in which the pKb difierence is less than two.

No'rE.Coating time 1 minute.

Generally, the composition is diluted to a 3 to 25% From Table 1, it is clear that the composition of the by weight solid content using water as a medium, and p ese invention ShOWS Superior P p maintained at a temperature of 5 to 50 C. The material The imp Strength Was determined y pp a Q to be coated is connected to the positive terminal of an h y having an end h in diametfif 0n the 1 electric source and (usually) the vessel itself is made into cotltmg film then meaSPTmg the maxlmum droPPlng the cathode The cathode can also provided in the height from WhlCh the coating surface was not damaged. vessel per se, and the material to be coated can be im- EXAMPLE 2 mFrsed therein, While immersing 30 to 300 volts of A zinc phosphate treated soft steel plate was coated by director pulsating current are passed through the system. the Same method as in Example 1, but with Substituting The coated material is then removed and, after being 05 part, 2 Parts, 5 parts, 10 parts and 20 Parts of treated appropriately: if necessary: heated to a Suitable line for 2 parts of pyridine. Although in all cases coatings temperature y 80 t0 to Cure the p having a glossy surface were obtained, 10 parts of 'y-picotioh- This is the Well known gene! at electrophoretic p line was best; 20 parts of 'y-picoline was almost equal; and sition method. The material to be coated is usually an for 5 parts, 2 parts and 0.5 part of 'y-picoline, the smoothelectroconductive material as a metal such as aluminum, ness of the surface changed, and in 0.5 part of -picoline zinc, iron, copper, various alloys, etc.; a surface processed unevenness was observed. The change was most noticeable metal such as almite; or zinc phosphate treated iron, etc. in the Order given- Actually, the body of a car, a car wheel, household elec- EXAMPLE 3 trical articles, etc. are typically treated. In the coating A li resin prepared b copolymerizing 10 parts film obtained y using the coating composition for an of acrylic acid, 30 parts of methyl methacrylate, 50 parts electro-deposition coating according to the present invenof butyl acrylate and 10 parts of hydroxyethyl methacrytion, not only is the flowability at curing excellent, but late in n-butanol having added 0.5 part of n-dodecylmercaptane and 1.0 part of azobisisobutyronitrile was water-solubilized with parts of diethyl-aminoethanol. Nikalac MW-22 (trademark) [a methoxy melamine resin, soluble in isopropyl alcohol and water, having a hue of below 30 and a Gardner viscosity of below A] was added as a melamine thereto at a ratio of 6 parts of acrylic resin to 4 parts of melamine. Titanium dioxide Taipaque R550 (trademark) [having a purity of 96.0%, a particle diameter of 0.25 to 0.40, a specific gravity of 4.2, a tinting strength of 1,700 and an oral absorption value of from 20 to 22] was dispersed in this resin solution at a ratio of 1 part of pigment to 3 parts of resin. This composition was diluted with water to a solid content to by weight. At this time, the pH was 8.2. 100 liters of this composition was placed into a rectangular stainless steel tank provided with an overflow recurring apparatus. The vessel was connected to the cathode terminal of an electric source. A zinc phosphate treated iron rectangular article to be coated was connected to the anode terminal of an electric source and was immersed into the composition, care being taken not to contact the article with the vessel. After adding 4% pyridine based on the solid content of composition, the composition was electrodeposited under the condition of 150 volts for one minute at 30 C. After washing with water, the coated article was cured at 160 C. for minutes. Upon comparison with the case of treatment in the same manner without adding pyridine on the surface state of coating, adding pyridine resulted in the coating surface being very smooth. In the case of not adding pyridine, the surface was very uneven.

EXAMPLE 4 8 parts of itaconic acid, 20 parts of methacrylic acid, 50 parts of butyl acrylate, 14 parts of styrene, and 8 parts of hydroxypropyl rnethacrylate were copolymerized in 80 parts of isopropyl alcohol under reflux using 0.5 part of azobisisobutyronitrile as an initiator. To this resin solution 3 parts of 3.5-lutidine and 6 parts of triethylamine were added, and water was added thereto to adjust the resin content to 50% at the time of heating to 110 C. 10 parts of Taipaque R550 (trademark) and 1 part of Mitsubishi carbon 100 (trademark) [carbon black] were added to 100 parts of this water-solubilized resin, and they were milled on a roll. The mixture was then diluted with water and adjusted in its pH with diethylaminoethanol so that the solid content was 10% and the pH was 9.0. The composition obtained was electrodeposited onto a zinc phosphate treated iron plate at 30 C. under the condition of 60 volts for two minutes. The plate was then cured at 190 C. for 30 minutes to obtain a glossy coating having a smooth surface.

EXAMPLE 5 1 part of triethanolamine and 7 parts of triethylamine were added to the acrylic resin obtained in Example 3 to water-solubilize the resin. Further 4 parts of 3.5-lutidine were added thereto, and then water was added thereto to adjust the resin content to about 50%, by weight. Thereafter, the resin was blended with Nikalac MX-40 (trademark [Nikalac MX-40 is a methoxybutoxy melamine resin having a Gardner viscosity of U-Y; a specific gravity of 1.05-1.25 at 20 C. and an acid value not exceeding 0.5] and Taique R550 (trademark) at a ratio of the acrylic resin to Nikalac MX to Taipaque R550 of 7 to 3 to 4. The mixture was milled on a three-roll mixer. The composition was diluted with water to a 15 by Weight solid content, and, thereafter, was adjusted in its pH with triethylamine to 9.0. The composition was electrodeposited on an iron plate treated with Bondelite #144 (trademark, zinc phosphate treating agent) at C. under 100 volts. After washing with water, the article was cured at 180 C. for 20 minutes to obtain a smooth coating having a gloss of about 75% at 60 mirror reflection. The surface of the coating showed good adhesiveness (100/100 in the cross-line test of the JIS), excellent impact strength (500 g., 50 cm., /2 inch), and excellent alkali resistance (immersion in 1% NaOH for one day).

In contrast, for the case of treatment in the same manner without adding 3.5-lutidine, the electrodeposited coating film tends to peel off under water washing, and the surface of coating obtained was very uneven so that its gloss was below 40% at 60 mirror reflection. The deposition time was two minutes, for both cases.

To further aid in an understanding of the present invention, when the acrylic polymerization takes place, usually a temperature within the range 50-170 C., is most preferred, this being about the boiling point of the solvent. Normal pressure is most preferably utilized, and the time of reaction may be from about 1 to about 72 hours.

To further expand upon the present invention, illustrative esters of a carboxylic acid having a co-polymerizable double bond, which enter into the acrylic polymerization, are alkyl esters having from 1 to 18 carbon atoms, such as methyl methacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, propyl methacrylate, octyl acrylate and the like.

What is claimed is:

1. An acrylic coating composition for electrodeposition consisting essentially of:

(1) an acrylic resin prepared by polymerizing from 3 to 20 parts by weight of a carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid, and itaconic acid, from 4 to 97 parts by weight of an alkyl ester of said carboxylic acid having from 1 to 18 carbon atoms and from 0 to 40 parts by weight of a material selected from the group consisting of acrylamide, styrene, and vinyl toluene.

(2) from 0.1 to 30 percent by weight, based on the weight of total solids in the composition, of a first organic amine compound selected from the group consisting of pyridine, picolines, lutidines and quinolines, and

(3) from 0.3 to 1.2 equivalents, to the carboxyl groups in the acrylic resin, of a second organic amine compound selected from the group consisting of mono-, di-, and tri-alkylamines, wherein the alkyl moiety of said alkyl amines has from 1 to 4 carbon atoms, diethylaminoethanol, triethanolamine, morpholine and piperidine,

said composition being diluted to a solids content of from 3 to 25 percent by weight, with water.

2. The acrylic coating composition of claim 1 wherein said acrylic resin is selected from the group consisting of a copolymer of styrene, butyl acrylate, hydroxyethylmethacrylate and itaconic acid, a copolymer of acrylic acid, methylmethacrylate, butyl acrylate and hydroxyethylmethacrylate, and a copolymer of itaconic acid, methacrylic acid, butyl acrylate, styrene and hydroxypropylmethacrylate.

3. The acrylic coating composition of claim 1 wherein said alkyl ester is selected from the group consisting of methylmethacrylate, methyl acrylate, ethyl acrylate, butyl acrylate, propylmethacrylate, and octyl acrylate.

4. The acrylic coating composition of claim 1 wherein said first organic amine compound is present in an amount of from 0.2 to 20%, by weight, based on the weight of total solids in the composition.

References Cited UNITED STATES PATENTS 3,403,088 9/1968 Hart 260-294 UA 2,918,391 12/ 1959 Hornibrook 260-29.4 UA 3,218,283 11/1965 Miller 260-32.4 X 3,506,601 4/ 1970 Sekmakas 26029.6 N UX ROBERT F. WHITE, Primary Examiner J. B. LOWE, Assistant Examiner US. Cl. X.R. 

